1. Technical Field of the Invention
The present invention relates to a method of build up welding to a thin-walled portion, and more particularly, a method repairing the tip portion of a moving blade of a gas turbine.
2. Description of the Related Art
FIG. 1 is a general external view showing a moving blade of a turbine. The moving blade is composed of blade portion 1, dovetail portion 2, and tip portion 3. Blade portion 1 has a blade-shaped section and generates rotational power from combustion gas. Dovetail portion 2 is located at an end of the blade portion 1, is fixed at a turbine disk not illustrated, and transmits rotational power to the turbine disk. The tip portion 3 has a thin-walled portion 3a that extends in the rotating direction. The tip of the tip portion 3 reduces leakage of gas as it closely faces the inner surface of the casing.
The moving blade of a gas turbine, with which high temperature and high pressure combustion gas collides, operates under very severe conditions. Consequently, thick-walled portion 3a at the tip of the moving blade may be subject to erosion of the wall or cracking due to abrasion and high temperature oxidation during operation. In such a case, the thin-walled portion is repaired by build up welding.
Conventional means of build up welding are already disclosed in patent references 1 and 2.
[Patent Reference 1]
Unexamined Japanese patent publication No. 171682, 1995
[Patent Reference 2]
Unexamined Japanese patent publication No. 180442, 1998
“Method of build up welding surface corner and build up welding jig” of Patent Reference 1 is shown in FIG. 2; before beginning the build up welding treatment, a build up welding jig 51 for cooling is placed in close contact with the corner surface side 53a of a welding workpiece 53, then build up welding is performed on the corner surface side 53b, and after weld beads 54 solidify, the build up welding jig 51 is detached from the welding workpiece 53.
“Method of repairing gas turbine blade” of Patent Reference 2 is shown in FIG. 3; when the tip of a gas turbine blade is repaired by build up welding, gas or fluid is passed through a ventilation hole inside the moving blade, during the repair operation. In FIG. 3, 61 represents the moving blade of a gas turbine, 63 build up welding portion, 64 the nozzle of a welding torch, 65 a welding electrode, 66 an arc, 68 an injection port of the gas or fluid, and 69 the gas or fluid.
Conventionally, a gas turbine moving blade is made mainly of a Ni base alloy. Such an Ni base alloy has a high tensile strength and extensibility, so one of its advantages is that defects are seldom produced as a result of the aforementioned build up welding.
Recently, on the other hand, studies and practice have led to a metal with a specific gravity smaller than that of the Ni base alloy (for instance, TiAl alloy) being used for gas turbine moving blades to improve the performance of the gas turbine. The specific gravity of the TiAl alloy is about one half that of the Ni base alloy, and is provided with higher toughness, so when TiAl alloy is used for gas turbine moving blades, the centrifugal force applied to the turbine disk by the moving blades installed can be sharply reduced and the gas turbine can rotate at a higher speed, which are advantages.
Table 1 compares the characteristics of the Ni base alloy and the TiAl alloy.
TABLE 1SpecificTensileSpecificCodeCompositiongravitystrengthtoughnessExtensibilityAlloyName(Wt %)(g/cm3)(MPa)(10E3m)(%)Ni baseIN100Ni-9.5Cr-15Co-3Mo-7.751014135alloy4.8Ti-5.5Al-0.015B-0.95V-0.06ZrTiA1K5CTi-31.2Al-7.0Nb-4683170.9alloy2.6Cr-1.4W-0.27B-0.06C
However, if build up welding is applied for a thin-walled portion in the same way as conventional methods, with moving blades made of TiAl alloy for the gas turbine, as shown in FIGS. 4A and 4B, there are problems such as molten metal dripping from a corner portion of thin-walled portion 3a and a dull corner portion.
To solve these problems, if a cooled jig is installed at the corner surface side as shown in Patent Reference 1, because of the small amount of extensibility and low tensile strength of TiAl alloy, there is a problem in that a weld crack is created by thermal stress at build up welding metal 5.
In FIGS. 4A and 4B, a and b show dull portion and weld crack portion, respectively.